Whether or not a secondary battery is degraded is commonly determined based on the output voltage of the secondary battery. For example, Japanese Patent Publication No. 1-33787 (PTL 1) discloses that an abnormal state of a battery system is detected based on the battery voltage when the engine is started. Specifically, when the engine is started at a low engine temperature and accordingly the battery voltage decreases, degradation of the performance of the battery is detected. As seen from the above, the battery is commonly inspected based on the output voltage.